The existence of a nation-wide twin register and central psychiatric register has made possible a catamnestic investigation of an unselected and representative sample of twins with manic-depressive disorders. From a total population of 11,288 same-sexed twin pairs born 1870-1920 in Denmark 126 probands from 110 pairs were ascertained. Among the co-twins of 69 monozygotic probands there were found 46 with manic-depressive disorders, and a further 14 had presented other psychoses or marked affective personality disorders or had committed suicide, yielding a proband rate of strict concordance, C1 = 0-67 and of broad, partial concordance, C2 = 0-87. The corresponding direct pairwise concordance rates were 32/55 = 0-58 and 46/55 = 0-84 respectively. For the dizygotic twins the proband concordance rate of C1 was 11/54 = 0-20 and of C2 20/54 = 0-37, and the direct pairwise rates were 9/52 = 0-17 and 18/52 = 0-35 respectively. The differences between the pairwise rates for the monozygotic and dizgotic twins are significant (P less than 0-001 at X2 analysis). This finding is in accordance with previous twin studies of manic-depressive disorders and confirms the evidence of a strong genetic factor. The concordance with respect to unipolar and bipolar forms was not in contradiction to recent evidence of a genetic difference between the bipolar and unipolar form, the latter probably related to the female sex.